Television and like camera tubes



April 16, 1963 E HENDRY TELEVISION AND LIKE CAMERA TUBES Filed Jan. 18, 1960 I lx/ /1/ 1/1 11'? III INVENTOQ ATTORNEYS United States atent 3,086,138 TELEVISION AND LIKE CAMERA TUBES Eric Douglas Hendry, Chelmsford, England, assignor to English Electric Valve Company Limited, London, England, a company of Great Britain Filed Jan. 18, 1960, Ser. No. 2,901 Claims priority, application Great Britain July 15, 1959 2 Claims. (Cl. 315-11) This invention, which is for improvements in or modifications of the invention forming the subect of co-pending application No. 818,186, now Patent No. 3,054,019, granted September 11, 1962, relates, like the said co-pending application, to systems including the television and like camera tubes generally known as image orthicon camera tubes. More specifically the invention relates to systems including image orthicon camera tubes of the kind in which, in order to ensure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, there is provided an electro-static field-terminating mesh which is mounted substantially parallel to and adjacent the target on the gun side thereof. -In the usual present day tubes of the kind referred to, this field-terminating mesh is electrically connected to the normally provided internal wall coating on the inside of the tube envelope. This field-terminating mesh is for the purpose of correcting the electro-static field in the neighbourhood of the target and increasing the strength of the declerating field in front of the target. This reduces or eliminates distortion due to deflection of the ray by the electrostatic field due to the charge pattern stored on the target.

As is explained in the co-pending application 818,186, operating defects occur with known image orthicon camera tubes of the kind referred to, because of secondary electron emission from the field-terminating mesh. One such defect is that such secondary electrons released from the mesh as a result of bombardment by electrons of the ray on its way to the target cause an undesirably high DC. output. Also a distorting out-of-phase signal occurs due to secondary electrons released from the mesh returning to the electron multiplier section of the tube. Since these electrons start from the mesh with very low velocity they will arrive at the input of the multiplier section a little later than (i.e. a little displaced in time with respect to) the main signal; in other words they cause an out-of-phase and therefore a distorting signal component to be included in the output from the tube. The invention of co-pending application No. 818,186, which seeks to eliminate or substantially reduce the above-mentioned defects, consists, in its broadest aspect, in providing means for establishing, in the neighbourhood of the entrance to the electron multiplier section of the tube, a potential surface which is negative with respect to the potential of the field-terminating mesh adjacent the target structure of the tube, whereby secondary electrons emitted by said mesh are substantially prevented from entering said multiplier section. In the preferred embodiments of the invention of co-pending application No. 818,186 the means for establishing the potential surface are constituted by a ring electrode (or its equivalent, such as a number of interconnected electrodes in a ring) mounted on the normally provided electron gun and multiplier section structure of the tube at the end of said structure nearer the target, and means for applying a predetermined operating potential to said ring electrode.

The present invention also seeks to eliminate or substantially reduce the above-mentioned defects but to do so without having to provide any electrodes additional to those already present in a known image orthicon tube of the kind referred to.

According to the system of this invention the fieldterminating mesh adjacent the target structure of an "ice image orthicon tube of the kind referred to i.e. of the kind in which, in order to ensure that the electron beam scanning the target shall always be incident thereon at right angles thereto in all positions of cathode ray deflection, there is provided an electro-static field-terminating mesh which is mounted substantially parallel to and adjacent the target on the gun side thereof, is provided with its own independent connection lead, extending to a point outside the envelope of the tube whereby a potential which is positive with respect to the potential of the first dynode of the electron multiplier section of the tube, may. be applied to said mesh. When such a positive potential of suitable value is applied to the mesh it results in the production of a negative potential gradient between the mesh and the electron multiplier section of the tube and thus prevents secondary electrons emitted from the mesh from reaching the said multiplier section. The normally provided wall coating of the tube (to which, in a known tube of the kind referred to the field-terminating mesh is directly connected) is, in using the present invention, operated in the normal way, i.e. adjusted in potential until the required beam focus is obtained. In a tube in accordance with this invention, however, such adjustment of wall coating potential is effected independently of the adjustment of the potential of the field-terminating mesh which has its own independent connection lead.

The invention is illustrated in the accompanying drawing which is a highly simplified schematic representation of a preferred embodiment of the invention. The drawing shows only those parts of the tube necessary to an understanding of this invention.

Referring to the drawing, an image orthicon tube of the kind referred to has an envelope 1 with a photocathode 2 on the inside of the end wall thereof. 3 represents the usual internal coating on the inner wall of the tube. This coating has its own connection lead 3 shown for simplicity as going straight through the envelope wall, by means of which a suitably adjustable potential may be applied to the coating to obtain beam focus. 4 is a fieldterminating mesh adjacent and parallel to the customary target structure conventionally represented at 5. This mesh has its own independent connection lead 4 which, again for simplicity, is represented as passing straight through the wall coating and envelope being, of course, insulated from the coating. In practice the leads 3' and 4 would probably be brought out of the tube envelope via the customary plug connector (not shown) at the neck end of the tube. The electron gun is at the end of the neck of the tube, its cathode being given the reference 6. Arranged around the gun, as in the usual way, is a secondary electron multiplier section comprising a number of dynodes conventionally represented at 7, the first dynode being referenced 7 The electron gun and electron multiplier section are parts of a single mechanical unit or structure as in the customary way.

In use a suitable potential, positive with respect to the first dynode 7 is applied to the mesh 4 via the lead 4 To give typical operating potentials by way of example only, the mesh 4 might be held at about 300 volts and the first dynode 7 at about 250 volts making the mesh about 50 volts positive to the first dynode. In typical present day known practice, with the mesh connected to the coating 3, the mesh would normally be about to 200 volts and the first dynode at about 250 to 300 volts with the result that secondary electrons from the mesh reach the electron multiplier section of the tube to produce the operating defects hereinbefore mentioned.

I claim:

1. A system including an image orthicon tube having an electron gun, a multiplier section adjacent said gun, said multiplier section including at least one dynode, a

target, and an electrostatic field terminating mesh mounted substantially parallel to and adjacent the target, whereby the beam scanning the target shall always be incident at right angles thereto in all directions of beam deflection,

said system further including a source of potential which is positive with respect to the potential of said dynode, and means to apply said positive potential to said field terminating mesh, the voltage from said source being of such magnitude as to establish a predetermined negative potential gradient between the mesh and said electron multiplier section, whereby secondary electrons emitted from the mesh are prevented from reaching said multiplier section.

2. A system including an image orthicon tube as set forth in claim 1, said tube having an electrically conductive coating on the inside of the envelope Wall of the tube, a connecting lead for said coating independent of the lead from the field terminating mesh, said lead having a connection outside the tube envelope, said system further including means to apply to the connection to said Wall coating a predetermined potential of such magnitude as to produce the required beam focus.

References Cited in the file of this patent UNITED STATES PATENTS 2,548,405 Synder Apr. 10, 1951 2,747,131 Sheldon Oct. 12, 1951 2,598,919 Jensen June 3, 1952 2,652,515 McGee Sept. 15, 1953 

1. A SYSTEM INCLUDING AN IMAGE ORTHICON TUBE HAVING AN ELECTRON GUN, A MULTIPLIER SECTION ADJACENT SAID GUN, SAID MULTIPLIER SECTION INCLUDING AT LEAST ONE DYNODE, A TARGET, AND AN ELECTROSTATIC FIELD TERMINATING MESH MOUNTED SUBSTANTIALLY PARALLEL TO AND ADJACENT THE TARGET, WHEREBY THE BEAM SCANNING THE TARGET SHALL ALWAYS BE INCIDENT AT RIGHT ANGLES THERETO IN ALL DIRECTIONS OF BEAM DEFLECTION, SAID SYSTEM FURTHER INCLUDING A SOURCE OF POTENTIAL WHICH IS POSITIVE WITH RESPECT TO THE POTENTIAL OF SAID DYNODE, AND MEANS TO APPLY SAID POSITIVE POTENTIAL TO SAID FIELD TERMINATING MESH, THE VOLTAGE FROM SAID SOURCE BEING OF SUCH MAGNITUDE AS TO ESTABLISH A PREDETERMINED NEGATIVE POTENTIAL GRADIENT BETWEEN THE MESH AND SAID ELECTRON MULTIPLIER SECTION, WHEREBY SECONDARY ELECTRONS EMITTED FROM THE MESH ARE PREVENTED FROM REACHING SAID MULTIPLIER SECTION. 